Whisker-reinforced ceramic cutting tool material

ABSTRACT

The present invention relates to a ceramic whisker-containing cutting insert for chipforming machining of steel having improved mechanical, thermal and wearing properties. 
     Besides an oxide-based matrix, the material contains 5-50, preferably 10-40, % by volume of homogeneously distributed whiskers or platelets of nitrides, carbides and/or borides of metals from group IVB (Ti, Zr or Hf) and/or VB (V, Nb or Ta) in the periodic system (or solid solutions thereof) having a linear expansion coefficient being lower than that of Al 2  O 3 , preferably at the most 85% of that for Al 2  O 3  measured at 300-1300K. The properties of the composite material can be further modified by coating the whisker material with thin layers. The whisker reinforced cutting tool material shows an improved toughness behavior.

This application is a continuation of application Ser. No. 07/598,115,filed Oct. 16, 1990, now U.S. Pat. No. 5,141,901.

The present invention relates to ceramic cutting tool materials andparticularly to such cutting materials in which monocrystalline whiskers(hair crystals) are uniformly distributed in a ceramic matrix containingaluminum oxide which leads to an improved strength and toughness withoutnegatively influencing the wear resistance.

Ceramic cutting tools have now been available for several decades butthey have not until recently had any great commercial importance for usein chipforming machining. The main reason for the limited growth ofceramic cutting tools has been sudden and unexpected tool failuresbecause of their inherent inadequate strength and toughness.

In recent years, the properties of ceramic cutting tool materials havebeen improved in many respects and their use in cutting of cast iron andhot-strength (e.g., nickel-base) alloys has relatively increased. Theproportion of ceramic cutting inserts used is still very small, however,where steel is the dominating work piece material being machined becausesteel makes large simultaneous demands upon strength, toughness and wearresistance which have not been fulfilled by currently known ceramiccutting tool materials.

Aluminum oxide based cutting tool materials are very sensitive tothermal crack formation because aluminum oxide in itself has arelatively poor thermal conductivity. This leads to very short toollives in machining steel, particularly under conditions with shortoperating times and varying cutting depth.

To a certain extent, the thermal properties have been improved byadditions of titanium carbide and/or titanium nitride which enhance thethermal conductivity of the composite material. The additional oftitanium carbide/nitride also increases the hardness of the material. Incomparison with pure aluminum oxide materials, an increased tool life istherefore obtained in the cutting of harder work piece materials and inoperations demanding thermal shock resistance. However, this kind ofmaterial has too poor a toughness behavior for general use in thecutting of steel.

A later development relates to alloying of uniformly dispersedfine-grained zirconium oxide particles in a matrix of aluminum oxide. Atransition of the "metastable" zirconium oxide particles during useincreases both strength and toughness and thus leads to a more predictedtool life.

The thermal properties of said type of material are, however, onlyslightly better than those of pure aluminum oxide materials. Therefore,initiation and growth of thermally induced cracks is still a greatproblem in practical cutting operations generating high cutting edgetemperatures such as the cutting of steel.

It has recently been shown that alloying of SiC-whiskers, withmono-crystalline hair crystals, in a matrix of aluminum oxide leads to agreatly improved fracture toughness and strength. Ceramic cutting toolmaterials based upon said concept have shown very good performances inthe cutting of hot-strength materials in particular but in the cuttingof steel they have shown surprisingly short lives because ofpreferential attack of the SiC-crystals. This leads to a weakening ofthe surface zone with accompanying high wear and risks of crackinitiation.

It is thus an object of the invention to obtain a ceramic material forchipforming machining, particularly of steel, in which theabove-mentioned weaknesses of today's known aluminum oxide based cuttingtool materials have been eliminated. Thus, in materials according to theinvention, there is a unique combination of strength, toughness,thermal-shock-resistance and wear resistance which has not been possibleto obtain with earlier known oxide-based cutting tool materials.

In U.S. Pat. No. 4,867,761, oxide-based ceramic cutting tool materialsare strengthened by whiskers of carbides, nitrides and borides of Ti orZr or solid solutions thereof having a low solubility in steel resultingin a cutting tool material with an improved and more predictabletoughness, as well as improved strength and resistance to thermal shockswithout deterioration of the wear resistance to any appreciable degreeparticularly when cutting steel. This has not been possible with earlierknown material compositions.

It has now been surprisingly found that further improvements of fracturetoughness and resistance to thermal shocks can be obtained if thedifferences in thermoelastic properties between the aluminum oxide andthe whisker material are large.

According to R. W. Rice, Ceram. Eng. Sci. Pro., 2 (1981) 661. and A. G.Evans, H. K. Bowen, R. W. Rice and K. M. Prevo, Rep. Materials ResearchCouncil Summer Conf., La Jolla, Calif., 1983. the most essentialcriteria in developing a fiber-strengthened ceramic material are asfollows:

1. A high fiber modulus and strength (preferably more than twice that ofmatrix for an effective load transition to the fiber);

2. A small fiber diameter (of the same range of order as the grain sizein the matrix);

3. A homogenous distribution of fibers;

4. No or small chemical bond between fiber and matrix;

5. No chemical reactions which destroy or change the properties of thefiber;

6. A large volume fraction of fibers; and

7. Comparable thermal expansion coefficients (alpha), but preferably agreater expansion coefficient for the fiber than for matrix so thatcompressive strains are obtained in the matrix.

Under point 7, it is specially mentioned that the thermal expansioncoefficient shall be greater than or equal to that of matrix. Thisrequirement is fulfilled in the above-mentioned U.S. Pat. No. 4,867,761because the difference of the thermal expansion coefficient between TiC,TiN, TiB₂ and ZrC, ZrN, ZrB₂, respectively, and Al₂ O₃ is very small.

It has now been found particularly favorable to use whiskers with alower thermal expansion coefficient than that of Al₂ O₃ which leads totensile strains in the aluminum oxide and compressive strains in thewhisker. The exact mechanisms of the improvements of fracture toughnessand resistance to thermal shock are not known but probably there is alsoa formation of microcracks during use which favorably affects thetoughness behavior and resistance to thermal shock. Another factor whichcan be important is that tensile strains in the aluminum oxide areoriented in parallel with the longitudinal axis of the whisker andtherefore crack propagation occurs perpendicular to the whisker whichfavors bridging of the cracks (the whisker bridges over a passing crackand strives to hold it together).

The ceramic cutting tool material according to the present inventioncomprises an oxide-based, preferably aluminum oxide-based matrix with5-50, preferably 10-40, often 25-35, % by volume of homogeneouslydispersed whiskers based upon nitrides, carbides and/or borides ofmetals from group IVB (Ti, Zr or Hf) and/or VB (V, Nb or Ta) in theperiodic system or solid solutions thereof, with the whiskers having athermal expansion coefficient which is lower than that of Al₂ O₃ and alow solubility in steel.

In Table 1 below, examples are given of whisker materials which fulfillthe criteria of low solubility in steel.

                  TABLE 1                                                         ______________________________________                                        Thermal expansion coefficient (10.sup.-6 K.sup.-1 300-1300                    ______________________________________                                        K)                                                                            Al.sub.2 O.sub.3                                                                              8.4          about 8                                          TiN             7.5-8.9                                                       HfC             6.5          about 6                                          HfN             6.3                                                           NbC             5-7                                                           TaC             4-6.5        about 5                                          TaN             5.0                                                           TaB.sub.2       5.2                                                           ______________________________________                                    

As evident from Table 1, no or very small self-strains can be expectedby the use of TiN-whiskers. Use of HfC, HfN and NbC gives tensionalstrains in the aluminum oxide These strains will be still greater by useof TaC, TaN or Tab₂.

The whisker material consists of monocrystals with a diameter of 0.2-10μm and a length of 2.5-100 μm and a length/diameter ratio that ispreferably 5-10. The grain size of the oxide matrix shall be <10 μm,preferably <4 μm. The oxide matrix is essentially ceramic oxides orceramic oxides mixed with hard carbides and/or nitrides and/or boridesand/or binder metal. Preferably, the ceramic matrix shall contain <20%by volume of ZrO₂.

The cutting tool material is made by wet-milling and mixing of theoxide-based powder and monocrystalline whisker crystals. After drying,the mixture is pressed to desired geometrical form and sintered withoutpressure to almost theoretical density. After sintering, the possibleremaining, so-called, closed porosity can be removed by hot-isostaticpressing. If it is not possible during sintering to obtain closedporosity, the material can be pressure-sintered by a suitable graphitetool or be hot-isostatically pressed after canning to desired density.The sintering method is dependent upon the whisker material and ischosen so that the cutting tool material reaches a density exceeding 99%of the theoretical density.

The use of whisker-reinforcement in the aluminum oxide-based matrixleads to a significant increase of the fracture toughness. Themechanisms causing said increase can be load transition between whiskerand matrix, crack deflection and whisker pull-out. Said mechanismsexploit and depend upon crack-propagation occurring along a sufficientlyweak interface between whisker and matrix. The bonding force betweenwhisker and matrix is therefore an important parameter which can beinfluenced by coating the whisker material with thin layers of, forexample, BN or graphite in order to further improve the fracturestrength. In the actual case, a contribution from the internal strainsbeing formed by use of whiskers with lower thermal expansion coefficientthan that of matrix is also obtained.

The invention is additionally illustrated in connection with thefollowing Example which is to be considered as illustrative of thepresent invention. It should be understood, however, that the inventionis not limited to the specific details of the Examples.

EXAMPLE

30% by volume of titanium nitride-, hafnium nitride- and tantalumnitride-whiskers, respectively, are wet-mixed with 70% by volume of amixture of 95.5% by weight of Al₂ O₃, 4.2% by weight of ZrO₂ and 0.3% byweight of MgO. After drying in vacuum, the mixtures are dry-mixed andpressed to bodies. The bodies are sintered at 1550° C. to 99% of thetheoretical density by isostatic hot-pressing.

K_(IC) is measured by means of the so called indentation method. In saidmethod an indentation is made by means of a pyramid diamond tip andK_(IC) is calculated from the size of the cracks produced from thecorners of the indentation.

At the measurement a reference of Al₂ O₃ +4.2% by weight of ZrO₂ +0.3%by weight of MgO was used.

K_(IC) -values for the various material combinations are given in Table2.

                  TABLE 2                                                         ______________________________________                                                                  K.sub.IC                                            ______________________________________                                        1.    Al.sub.2 O.sub.3 + 4.2% by weight of ZrO.sub.2 + 0.3%                                                   6.1                                                 weight of MgO + 30% by volume of TiN                                          (whiskers)                                                              2.    Al.sub.2 O.sub.3 + 4.2% by weight of ZrO.sub.2 + 0.3%                                                   7.0                                                 weight of MgO + 30% by volume of HfN                                          (whiskers)                                                              3.    Al.sub.2 O.sub.3 + 4.2% by weight of ZrO.sub.2 + 0.3%                                                   8.2                                                 weight of MgO + 30% by volume of TaN                                          (whiskers)                                                              4.    Al.sub.2 O.sub.3 + 4.2% by weight of ZrO.sub.2 + 0.3%                                                   4.0                                                 weight of MgO                                                           ______________________________________                                    

From Table 2, it is evident that while alloying with TiN-whiskersincreases the fracture toughness over the non-whisker-reinforcedmaterial, even greater increases can be obtained using HfN- andTaN-whiskers. The fracture toughness increases with an increasingdifference in the thermal expansion coefficient of the whiskers. Thepreferred embodiment therefore includes a whisker material having anexpansion coefficient of at the most 85%, preferably at the most 70%, ofthat of Al₂ O₃ measured in the temperature range 300-1300K. The fracturetoughness is a parameter which shows the capacity of a material to standmechanical stresses without leading to catastrophic failures. In thecase of chip-forming machining, it means that higher feeds can bepermitted, i.e., the rate of material removal can be increased for acertain cutting speed.

The invention has been described by referring to whisker-reinforcedmaterials. It is obvious to those skilled in the art, however, thatsimilar results can be obtained if the whiskers are wholly or partlysubstituted by monocrystalline platelets (discs) or similar. Thediameter of the platelets shall be less than 50 μm, preferably less than20 μm, and the thickness less than 5 μm, preferably less than 2 μm.

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the invention.

What is claimed is:
 1. Oxide-based ceramic cutting insert forchipforming machining of steel, comprising a ceramic oxide-based matrixof alumina and less than 20% zirconia and 5-50% by volume, ofhomogeneously dispersed whiskers or platelets of at least one of thecarbides of in the periodic system Nb, Hf and V and the nitride of Hf,the whiskers or platelets having a linear expansion coefficient measuredat 300-1300K which is at the most 85% of that of Al₂ O₃.
 2. Cuttinginsert according to claim 1, wherein the oxide-based matrix contains10-40% by volume of the said whiskers or platelets.
 3. Cutting insertaccording to claim 1, wherein the whiskers or platelets have a linearexpansion coefficient measured at 300-1300K, which is at the most 70% ofthat of Al₂ O₃.
 4. Cutting insert according to claim 1, wherein theoxide-based matrix contains 25-35% by volume of the said whiskers ofplatelets.
 5. Cutting insert according to claim 1, wherein the whiskersor platelets are HfC, HfN, or NbC.